Metals subjected to high neutron fluxes in high temperature environments, such as exists in fast breeder reactors and controlled thermonuclear reactors, undergo a phenomena known as swelling. Swelling produces volume expansion in components which has serious implications for the design of these components.
Energy costs increase due to swelling. A cost-benefit analysis conducted by P. R. Huebotter et al, USAEC Report ANL-7786, Argonne National Laboratory, 1971, indicated that having to accommodate only 5% swelling in breeder reactors over the period of 1970 to 2020 has a 1970 present worth of from $864 million to $5.6 billion relative to the case of 15% swelling. Hence, even small successes in the control of swelling yield enormous cost savings.
Consequently, many different techniques have been suggested to reduce swelling. Prevention of swelling was attempted by cold working of 316 stainless steel used in the Fast Flux Test Facility (FFTF). This has the disadvantage that cold work recovery occurs at temperatures near 0.6 of the absolute melting point of the material. Further, cold work may not be homogeneously distributed throughout the material, thus leaving certain areas where swelling may occur.
Another method attempted was the development of a fine dispersion of precipitates which acts as sinks for vacancies causing swelling. This principle has been demonstrated by a nickel-base alloy developed by the British. It is, however, subject to over-aging of the precipitate at elevated temperatures and instability of the precipitate in high neutron fluxes.
Mechanical solutions have been attempted as well. These involve the use of articulated, flexible or undercut ducts, replaceable structural components, etc. Attempts have also been made to reduce neutron energy by introducing core moderators. Neither has been highly successful.